The precise positioning of an object is required in many applications, including lithography processing in semiconductor manufacturing for forming integrated circuits on semiconductor wafers. As the circuit density of integrated circuits increases and feature size decreases, the accuracy in the methods for laying down the circuits on the semiconductor wafer must improve. Various systems and methods have been developed to attempt to improve positioning and movement of a semiconductor wafer in the lithography process. One way to increase the accuracy is to reduce system complexity and size of the stage device, thus providing greater stability of motion during positioning of the wafer.
Air bearing systems are often used to provide smooth and accurate movement between a stage and another planar surface or a guide structure. An example of a stage device for use in semiconductor processing equipment is disclosed in U.S. Pat. No. 5,760,564. The stage assembly includes two guide rails, one movable in the X direction and the other movable in the Y direction. A plurality of air bearings are attached to the guide rails and stage for movement of the guide rails and stage relative to the base. Since the bearings are attached to the stage and travel with the stage, the base must be at least as large as the diameter of the bearing plus the entire stroke (travel) of the stage. This results in a large base and stage.
Examples of air bearing systems include differentially pumped air bearing systems such as those disclosed in U.S. Pat. No. 4,191,385 to Fox and U.S. Pat. No. 4,425,508 to Lewis et al.. Lewis et al. disclose an air bearing intended for use in a vacuum chamber, such as in an electron beam lithography system. The air bearing includes an air bearing plate defining a plurality of H-shaped grooves around its outer periphery and a metering valve disposed in each H-groove for introducing pressurized air into each H-groove. The pressurized air provides an air cushion between the face of the air bearing plate and the opposing face of a moving plate. The air bearing plate also includes a central vacuum region circumscribed by two concentric pump-out slots. The pressure in the vacuum region is maintained by conventional vacuum pumps. The two concentric pump-out slots are radially inward of the H-grooves to scavenge the air escaping inwardly from the H-grooves in order to prevent the air from reaching the vacuum region. Thus, the pump-out slots separate the air bearing from the central vacuum region.
Air from the air bearing flows through the small gap between the air bearing plate and the opposing face of the moving plate. Small values for the gap are required in an attempt to reduce air flow to the vacuum region. However, the smaller the gap, the tighter the necessary mechanical tolerance on the air bearing plate and the opposing surface of the moving plate, substantially increasing the manufacturing costs. A typical air bearing gap is approximately 5 microns, requiring precision machining over a relatively large surface area to substantially increase manufacturing costs, particularly because two such large precision machined surfaces are needed. In addition, because the stiffness of the air bearing is a function of this gap, adjusting the gap purely to control the air flow is often impractical.
Thus, it is desirable to provide an air bearing which is effective in preventing gas leakage into the vacuum environment and which is suitable for use within a vacuum environment and not just surrounding a vacuum. It is further desirable to provide such an air bearing which is cost effective, simple to manufacture, robust, and which produce very little wear or vibration during operation.